Contrast adjusting circuit

ABSTRACT

A contrast adjusting circuit used in a PDP display prevents the image from being whitish by suppressing the rise of the luminance at a low level when the contrast is heightened and optimally adjusts the contrast of the images always varying with time. The circuit contains an average luminance calculating section ( 12 ), an LUT ( 14 ), and an output video data calculating section ( 16 ), wherein the average luminance calculating section ( 12 ) determines the average luminance level APL for n frame images from the input video data X (X&gt;=0) on the X-Y coordinate system, the LUT ( 14 ) determines central value data Xc, Yc from the APL, and the output video data calculating section ( 16 ) determines the output video data Y (Y&gt;=0) collected after the contrast is adjusted according to the calculation formula Y=A·X+Yc−A·Xc where A (A&gt;0) is the slope variably set for contrast adjustment, whereby the rise of the luminance at a low level when A is increased and the contrast is heightened, is suppressed to adjust the contrast corresponding to the APL for frame images.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a contrast adjusting circuit fordisplays such as a PDP, using the PDP (Plasma Display Panel) or thelike.

BACKGROUND ART

Conventionally, this kind of contrast adjusting circuit is composed of amultiplying circuit 10 as is shown in FIG. 1, the multiplying circuit 10being designed to obtain the output video data Y by multiplying thevideo data X by a variably set slope A.

When the input video data X and the output video data Y are representedby X (X>=0) and Y (Y>=0) of the X-Y coordinates, there holds arelationship, Y=A X, an expression representing a straight line Aincluding the original (0, 0), between the input video data X and theoutput video data Y as shown in FIG. 2. That is, assuming that thevariable ranges of the slope A to be from A1 to Am (A1=<A=<Am, m beingan integer 2 or more), the contrast increases when the slope A isincreased (e.g., Y=Am·X), whereas the contrast decreases when the slopeA is decreased (e.g., Y=A1·X).

However, in the case of the prior art as is shown in FIG. 1 there occursa problem such that, when the contrast is set to a high level by settingthe slope A to a high level (e.g., A=Am), the luminance of the imagecorresponding to low-level input video data X (i.e., low input level)becomes too high causing the image to become whitish.

Another problem of the prior art has been that the optimal contrastadjustment cannot be made to the ever-changing conditions of the image.

The present invention is made in consideration of the problem of theprior art and is first intended to provide a contrast adjusting circuitcapable of controlling the increase in low-level luminance at the timewhen the contrast is increased, thereby preventing the image frombecoming whitish.

The second object of the present invention is to provide a contrastadjusting circuit capable of adjusting the contrast to an optimal levelcorresponding to the ever-changing conditions of the image.

DISCLOSURE OF THE INVENTION

The contrast adjusting circuit according to the present inventioncomprises a video data calculating section designed to obtain the outputvideo data by substituting the input video data X, the slope A and thecentral point data Xc and Yc in the calculation formula, Y=A·X+Yc−A·Xcrespectively, where X (X>=0) of the X-Y coordinate system is the inputvideo data; Y (Y>=0) is the output video data with adjusted contrast; A(A>0) is the variably set contrast adjusting slope; Xc and Yc (Xc>0,Yc>0) are the central point data.

In such a system, the calculation formula Y=A·X+Yc−A·Xc to be applied tothe operation of the video data calculating section produces a straightline passing through the points (Xc, Yc) within the first quadrant(Xc>0, Yc>0) not including the origin (0, 0) of the X-Y coordinates.Therefore, when the slope A, the slope for adjusting the contrast, isincreased to increase the contrast, the rise of the output video data Ycorresponding to the low level of the input video data X can beprevented, unlike the case of the prior art represented by thecalculation formula, Y=A·X, which produces a slope passing through theorigin (0, 0), thereby preventing the image from becoming whitish.

Differing from the invention described above, in order to enable thecontrast to be adjusted according to the average luminance level of eachimage, there is also provided an average luminance calculating sectionfor obtaining the average luminance level of the images during ann-frame period (n being an integer of 1 or more), and a data convertingsection for converting the average luminance level to the correspondingcentral point data, Xc and Yc, out of a plurality of predeterminedcentral point data, so that the central point data, Xc and Yc, can beused as the central point data in the video data calculating section. Inother words, since the output video data Y, corresponding to the averageluminance level of the n frame images, can be obtained by thecalculation formula, Y=A·X+Yc−A·Xc, the contrast can be adjustedaccording to the level of the average luminance of the image.

Differing from the invention described above, in order to simplify thecomposition of the video data calculating section, the video datacalculation section comprises a subtracting circuit for subtracting thecentral point data Xc from the input video data X, a multiplying circuitfor multiplying the output data of the subtracting circuit by the slopeA, and an adding circuit for adding the central point data Yc to theoutput data of the multiplying circuit to thereby obtain the outputvideo data Y.

Differing from the invention described above, the central point data, Xcand Yc, are given as data on the basis of Y=X, and the video datacalculating section is designed to obtain the video data Y by using thecalculation formula, Y=A X+Xc·(1−A), which is introduced by settingYc=Xc in the calculation formula, Y=A·X+Yc−A·Xc. In this method, in allthe cases where the slope A for the adjustment of the contrast is 1 ormore, the rise of the luminance of the output video data Y at the lowlevel of input video data X can be prevented. In other words, the linerepresented by Y=A·X+Xc·(1−A), which is introduced by setting Yc=Xc inthe calculation formula, Y=A·X+Yc−A·Xc, passes through the pointrepresented by Y=X, so that, when A is 1 or more, the value of Xkrepresenting the X-coordinate value of intersection (Xk, 0) between thestraight line X and the straight line Y becomes larger than 1, therebynot only enabling the Y-coordinate values, corresponding to X-coordinatevalues ranging from 0 to Xk to be made equal to 0 but also enabling thevalue of Y to be made smaller than that by the prior art, even when thevalue of X is larger than Xk.

Differing from the invention described above, in order to enable thecontrast to be adjusted according to the level of the average luminanceof each image, the average luminance calculating section for obtainingthe average luminance level of the images during the n-frame period onthe basis of the input video data X and the data converting sections forconverting the average luminance level, which is obtained by the averageluminance calculating section, to the corresponding central point data,Xc and Yc, out of a plurality of predetermined central point data,whereby the central point data, Xc and Yc, which are outputted from thedata converting section, are processed by the video data calculatingsections.

In other words, the central point data, Xc and Yc, which are to beprocessed by the video data calculating section, undergoes the contrastadjustment as being the data corresponding to the average luminancelevel of the n-frame images.

Differing from the invention described above, in order to simplify thecomposition of the video data calculating section, the video datacalculating section comprises a first multiplying circuit formultiplying the input video data by the slope A, a subtracting circuitfor subtracting the slope A from the constant 1, a second multiplyingcircuit for multiplying the output data of the subtracting circuit bythe central point data Xc, and an adding circuit for adding the outputdata of the first multiplication circuit to the output data of thesecond multiplying circuit to thereby obtain the output video data Y.

Differing from the inventions described above, in order to simplify thecomposition of the data converting section, the data converting sectioncomprises a look-up table which outputs the corresponding central pointdata, Xc and Yc, by using the average luminance, which is obtained bythe average luminance calculating section, as the address.

Differing from the invention described above, in order to simplify thecomposition of the data converting section, the data converting sectioncomprises a look-up table which is designed to output the correspondingcentral point data Xc by using as the address the average luminanceobtained by the average luminance calculating section.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating the prior art.

FIG. 2 is a diagram illustrating the function of the system illustratedin FIG. 1, and is also an input/output characteristic diagram indicatingthe relationship between X and Y.

FIG. 3 is a block diagram illustrating the contrast adjusting circuit asthe first embodiment of the present invention.

FIG. 4 is a characteristic diagram illustrating the function of thesystem illustrating in FIG. 3, and is also an input/outputcharacteristic diagram illustrating the relationship of X and Y wherethe central point data Xc and Yc are the coordinates of Y=X; APL isconstant; and A is variable.

FIG. 5 is a characteristic diagram illustrating the function of thesystem illustrated in FIG. 3 and is also an input/output characteristicdiagram illustrating the relationship between X and Y where the centralpoint data (Xc, Yc), (Xcl, Ycl) and (Xcm, Ycm) are on the coordinates ofY=X; A, which is larger than 1, is constant; and the relationshipbetween X and Y where the APL is variable.

FIG. 6 is a characteristic diagram illustrating the function of thesystem shown in FIG. 3, and is also an input/output characteristicdiagram indicating that the central point data (Xc, Yc), (Xcl, Ycl) and(Xcm, Ycm) are represented by coordinates of Y=X, and that A which issmaller than 1 is constant, while the APL is variable.

FIG. 7 is a characteristic diagram illustrating the function of thesystem shown in FIG. 3, and is also an input/output characteristicdiagram showing the case where the central point data (Xc, Yc), (Xcl,Ycl) and (Xcm, Ycm) are represented by Y=aX+b where APL is constantwhile A is variable, and the case where the central point data, (Xc,Yc), (Xcl, Ycl) and (Xcm, Ycm) are represented by Y=aX+b where A is aconstant value larger than 1 while APL is variable.

FIG. 8 is a block diagram showing the contrast adjusting circuit as thesecond embodiment of the present invention.

BEST MODE OF CARRYING OUT THE INVENTION

The embodiments of the present invention will be described referring tothe drawing.

FIG. 3 shows a contrast adjusting circuit as the first embodiment of thepresent invention. In FIG. 3, reference numeral 12 represents theaverage luminance calculating section; 14, the LUT (Look Up Table) as anexample of the data converting section; 16, the video data calculatingsection.

As described later, on the X-Y coordinates, the X (X>=0) is the inputvideo data; the Y(Y>=0) is the output video data whose contrast has beenadjusted; A (A>0) is a predetermined variable slope for the adjustmentof the contrast.

The average luminance calculating section 12 determines the averagepicture level (hereinafter referred to as APL) of the images of ann-frame period (n being an integer larger than 1, e.g., n=1) on thebasis of input video data X.

The LUT 14 outputs the corresponding central point data, Xc and Yc(Xc>0, Yc>0), out of a plurality of predetermined central point data onthe coordinates Y=X.

The video data calculating section 16 substitutes the input video dataX, the slope A and the central point data (Xc, Yc) in the calculationformula Y=A·X+Yc−A·Xc to calculate the output video data Y.

The above video data calculating section 16 comprises a subtractingcircuit 18 for subtracting the central point data Xc from the inputvideo data X, a multiplying circuit 20 for multiplying the output dataof the subtracting circuit 18 by the slope A, and an adding circuit 22for adding the central point data Yc to the output data of themultiplying circuit 20 to generate the output video data Y.

Next, the function of the system illustrated in FIG. 3 will be describedreferring to FIGS. 4 through 7.

(1) When the input video data X is inputted to the average luminancecalculating section 12, the average luminance calculating section 12determines the APL of the images during an n-frame period (e.g., 1 frameperiod) on the basis of the input video data X.

(2) When the APL, obtained by the average luminance calculating section12, is inputted to the LUT 14, the corresponding central point data Xcand Yc are outputted out of a plurality of predetermined central pointdata represented by the coordinates of Y=X by using the APL as theaddresses.

(3) The video data calculating section 16 obtains the output video dataY by substituting the central point data, Xc and Yc, outputted from theLUT 14, in the calculation formula, Y=A·X+Yc−A·Xc, for each pixel (e.g.,for each dot) during an n-frame period, by using the input video data X,the set slope A and the APL of the immediately preceding n-frame imageas addresses.

In other words, Xc is subtracted from X in the subtracting circuit 18;the output data from the subtracting circuit 18 is multiplied by A inthe multiplying circuit 20; Yc is added to the output data A·(X−Xc) ofthe multiplying circuit 20 in the adding circuit 22; the obtained data{A·(X−Xc)+Yc} represents Y.

(4) When the slope A is varied for an image whose APL is constant, e.g.,when the user varied A within the range of A1 to Am by varying theresistance value for contrast adjustment, the contrast can be adjustedalong the straight line, Y(1)-Y(M) within the range indicated by thearrows in FIG. 4.

(4-1) When A is set larger than 1 (e.g., A=Am) to raise the contrast,the relationship between X and Y, for instance, becomes like Y(M) inFIG. 4. Y(M) represents the straight line, Y(M)=Am·X+Yc−Am·Xc, which isobtained by substituting Am for the A in Y=A·X+Yc−A·Xc. Therefore,regardless of the values of the central point data, Xc and Yc, in thefirst quadrant of the X-Y coordinate system where Y=X, the Xk of theintersection (Xk, 0) between the straight line Y(M) and the X-axis,becomes larger than 0 as shown in FIG. 4; when the X ranges from 0 toXk, Y can be set to 0; even when X is larger than Xk, the value of Y canbe set smaller than that in the case of the prior art (Y=Am·X).

In consequence, not only the rise of the luminance of the lower level atthe time when the contrast is raised thereby to prevent the image frombecoming whitish but also the contrast can be adjusted corresponding tothe APL of each image.

(4-2) When A is set smaller than 1 (e.g., A=A1), the relationshipbetween X and Y becomes, for instance, as Y(1) in FIG. 4. This Y(1)represents the straight line Y(1)=A1·X+Yc−A1·Xc, wherein A1 issubstituted for A in Y=A·X+Yc−A·Xc.

(5) When, after raising the contrast by setting A larger than 1 (e.g.,A=Am), A is set constant, and the APL is made to vary within the rangewhere the APL is APL1 or more but APLm or less, the contrast is adjustedalong the straight lines, Y(M1)−Y(Mm), within the range indicated byarrows in FIG. 5.

(5-1) When the level of the APL is relatively high (e.g., APL−APLm), therelation between X and Y becomes Y(Mm) as is shown in FIG. 5. This Y(Mm)represents a straight line, Y(Mm)=Am·X+Ycm−Am·Xcm, where Am issubstituted for, and Xcm and Ycm are substituted for Xc and Yc inY=A·X+Yc−A·Xc. In this case, Xcm and Ycm represent the data outputtedfrom the LUT14 by using the APLm as an address.

(5-2) Where the value of the APL is relatively small (e.g., APL=APL1),the relationship between X and Y becomes Y(M1) in FIG. 5. This Y(M1)represents a straight line, Y(M1)=Am·X+Ycl−Am·Xcl introduced bysubstituting Am for A, while substituting Xcl and Ycl for Xc and Yc, inY=A·X+Yc−A·Xc. In this case, Xcl and Ycl represent the data outputtedfrom the LUT 14 by using the APL1 as the address.

(5-3) Therefore, as shown in FIG. 5, the values, X1, Xk and Xm of theintersections, (X1,0), (Xk,0), (Xm,0) of the straight lines, Y(M1), Y(M)and Y(Mm) to the X-axis are respectively larger than 0, so that, wherethe value of X is within the range of 0, X1, Xk and Xm, the value of Ycan be set to 0 where the value of X is within 0, X1, Xk and Xm, and thevalue of Y can be made smaller than that in the case of the prior art,even when the value of X is X1 or more, Xk or more and Xm or more.

In consequence, not only the rise of the luminance for a lower level atthe time when the contrast is raised can be prevented to prevent theimage from becoming whitish but also the contrast of each image can beadjusted according to the APL of each image.

(6) Further, when, after setting A smaller than 1 (e.g., A=A1) to lowerthe contrast, and when A is constant, and when the APL is varied withina range, which is APL1 or more and APLm or less, the contrast isadjusted within the range indicated by the arrows and along the straightlines, Y(11) through Y(1m) in FIG. 6.

(6-1) When the value of APL is relatively high (e.g., APL=APLm), therelationship of X and Y becomes Y(1m) as shown in FIG. 6. This Y(1m)represents the straight line Y(1m)=A1·X+Ycm−A1·Xcm introduced bysubstituting A1 for A, and Xcm and Ycm for Xc and Yc in Y=A·X+Yc−A·Xc.

(6-2) When the APL is relatively small (e.g., APL=APL1), therelationship of X and the Y becomes as represented by Y(11) in FIG. 6.This Y(11) represents a straight line Y(11)=A1·X+Ycl=A1·Xcl, introducedby substituting A1 for A and Xcl and Ycl for Xc and Yc in Y=A·X+Yc−A·Xc.

(6-3) Thus, it is possible to adjust the contrast according to the APLof each image.

In the embodiment of the present invention shown in FIG. 3, the systemis designed so that the central point data, Xc and Yc, outputted fromthe LUT 14 correspond to the values represented by Y=X (i.e., Yc=Xc) byusing the APL as the address, but the present invention is not limitedto this embodiment. For instance, as shown in FIG. 7, the presentinvention is also applicable to the case where the system is designed sothat the central point data, Xc and Yc, outputted from the LUT 14 byusing the APL as the address, are set as being the values represented byY=a·X+b (i.e., Yc=a·Xc+b). In this case, a and b represent the properlyset constants respectively.

For instance, when the APL is kept constant while A is varied, as shownin FIG. 7, the contrast is adjusted within the range indicated by thearrows followed by a dot-and-dash line and along the straight line, Y(1)through the straight line Y(M).

Therefore, when A is increased to 1 or more (e.g., A=Am) to raise thecontrast, the relationship between X and Y is represented by the solidline Y(M) in FIG. 7. Even when the central point data, Xc and Yc, arerepresented by Y=aX+b, the value of the intersection Xk (Xk, 0) betweenthe straight line Y(M) and X-axis becomes larger than 0 within the rangeof A>1. That is, similarly to the case shown in FIG. 4, where X rangesfrom 0 to Xk, Y can be set to 0, and further even where X is Xk or more,the value of Y can be made smaller than that in the case of the priorart.

Therefore, not only the rise of the luminance at low levels at the timewhen the contrast is raised can be prevented, thereby preventing theimage from becoming whitish but also the contrast can be adjustedaccording to the APL of each image.

Further, when A is set as a constant value larger than 1 (e.g., A=Am)while letting the APL vary (APL1=<APL=<APLm), the contrast can beadjusted, for instance, along the straight lines, Y(M1), Y(M) and Y(Mm),based on the central point data, (Xcl, Ycl), (Xc, Yc) and (Xcm, Ycm)which are represented by Y=a·X+b.

Thus, when the APL is given as APL1, APLk or APLm, the relationshipbetween X and Y are represented by Y(M1), Y(M) and Y(Mm) shown in FIG.7, and, even when the central point data, (Xcl, Ycl), (Xc, Yc), (Xcm,Ycm) are represented by Y=aX+b, the values, X1, Xk and Xm at theintersections, (X1, 0), (Xk, 0), (Xm, 0), of the straight lines, Y(M1),Y(M) and Y(Mm), and the X-axis, become larger than 0. In other words,similarly to the case shown in FIG. 5, where the value of X is given as0, X1, Xk or Xm, Y can be made 0; even when the value of X is largerthan X1, Xk or Xm, the value of Y can be made smaller than that in thecase of the prior art.

In consequence, even if the contrast is raised, the rise of theluminance at a low level can be prevented, thereby not only preventingthe image from becoming whitish but also enabling the contrast to beadjusted according to the APL of each image.

FIG. 8 shows a contrast adjusting circuit as a second embodiment of thepresent invention, which is made available by replacing the LUT 14 andthe video data calculating section 16 of the first embodiment with theLUT 14 a and the video data calculating section 16 a, which will bedescribed later.

In FIG. 8, the descriptions of those reference numerals similar to thosein FIG. 3 will be omitted as being those for similar parts.

In FIG. 8, 14 a represents the LUT as an example of a data convertingsection, and 16 a represents an image data calculating section.

The LUT 14 a outputs a corresponding central point data Xc, out of aplurality of predetermined central point data represented by the X-Ycoordinate system for Y X, by using, as the address, the APL determinedby the average luminance calculating section 12.

The video data calculation section 16 a calculates the output video dataY by substituting the input video data X, slope A and central point dataX in the calculation formula, Y=A·X+Xc (1−A). This video datacalculating section 16 a performs the calculation, which is equivalentto the calculation by the calculation formula Y=A·X+Yc−A·Xc where Yc=Xc.

More specifically, the video data calculating section 16 a comprises afirst multiplying circuit 24 for multiplying the input video data X bythe slope A, a subtracting circuit 26 for subtracting the slope A fromconstant 1, a second multiplying circuit 28 for multiplying the outputdata of the subtracting circuit 26 by the central point data Xc, and anadding circuit 30 for adding the output data of the first multiplyingcircuit 24 and the output data of the second multiplying circuit 28 toproduce the output video data Y.

Next, the function of the system illustrated in FIG. 8 will be describedreferring to FIGS. 4 through 6.

(1) Upon input of the video data X to the average luminance calculatingsection 12, the APL of the image for an n-frame period (e.g., 1 frameperiod) is obtained by the average luminance calculating section 12 onthe basis of the input video data X.

(2) Upon input of the APL, obtained by the average luminance calculatingsection 12, to the LUT 14 a, the corresponding central point data Xc,out of a plurality of predetermined central point data represented bythe X-Y coordinate system for Y=X, is outputted from the LUT 14 a.

(3) During the n-frame period, the video data calculating section 16 aobtains the output video data Y for each pixel (e.g., for each dot) bysubstituting the input video data X, set slope A and the central pointdata Xc outputted from the LUT 14 a, in the calculation formula,Y=A·X+Xc·(1−A) by using, as an address, the APL of the immediatelypreceding n-frame image.

In other words, X is multiplied by the slope A in the first multiplyingcircuit 24; A is subtracted from the constant 1 in the subtractingcircuit; the output data (1−A) of the subtracting circuit 26 ismultiplied by Xc in the second multiplying circuit; the output data{Xc·(1−A)} of the second multiplying circuit 28 is added to the outputdata A·X of the first multiplying circuit 24 by the adding circuit 30,and the obtained data, {A·X+Xc·(1−A)} is treated as Y.

(4) In the case of an image whose APL is constant, when the slope A isvaried, similarly to the case of the embodiment given in FIG. 3, thecontrast is adjusted within the range indicated by the arrows and alongthe straight lines Y(1) through Y(M).

In other words, when the contrast is raised by increasing the value ofA, which is variable within A1 and Am, the relationship between X and Yis represented, for example, by Y(M) in FIG. 4, whereby the luminance atlow levels can be prevented from rising to prevent the image frombecoming whitish, while enabling the contrast to be adjusted matchingwith the APL of each image.

(5) When A is set larger than 1 (e.g., A=Am) and constant, while settingthe APL (APL1=<APL=<APLm) variable, the contrast can be adjusted withinthe range indicated by the arrows and along the straight lines Y(M1)through Y(Mm) as shown in FIG. 5. In other words, the relationshipbetween X and Y becomes as represented by Y(Mm) in FIG. 5 when the valueof the APL is relatively large, and it becomes as represented Y(M1) inthe same figure when the value of APL is relatively small. Therefore,when the contrast is raised, not only can the rise of the luminance atlow levels be prevented to prevent the image from becoming whitish butalso the contrast can be adjusted, matching with the APL of each image.

(6) When A is set smaller than 1 (e.g., A=A1) and constant, while theAPL (APL1=<APL=<APLm) is set variable, the contrast is adjusted withinthe range indicated with the arrows and along the straight lines Y(11)through Y(1m) as in FIG. 6.

In other words, the relationship between X and Y becomes like Y(1m) inFIG. 6 when the value of the APL is relatively large, while the samebecomes like Y(11) in the same figure when the APL is relatively small,thereby enabling the contrast to be adjusted, matching with the APL ofeach image.

In each of the embodiments shown in FIG. 3 and FIG. 8, the dataconverting section is composed of the LUT in order to simplify thecomposition of the data converting section. The present invention,however, is not limited to these embodiments, and thus the dataconverting section may be any one capable of converting the averageluminance, obtained by the average luminance calculating section, to Xcand Yc (Xc>0, Yc>0) out of a plurality of predetermined central pointdata represented by the X-Y coordinate system, or any one capable ofconverting the average luminance, obtained by the average luminancecalculating section, to the corresponding central point data Xc (Xc>0)out of a plurality of predetermined central point data represented bythe X-Y coordinate system.

In those previously described embodiments shown in FIG. 3 and FIG. 8, inorder for the contrast to be adjusted according to the average luminancelevel of each image, there are provided the average luminancecalculating section, designed to obtain the average luminance level ofthe images during the n-frame period based on the input video data X,and the data converting section, designed to convert the averageluminance level to corresponding central point data Xc, Yc or Xc out ofa plurality of predetermined central point data. The present invention,however, is not limited to these embodiments. That is, the presentinvention is also applicable to the case where the average luminancecalculating section and the data converting section are omitted so thatthe central point data, Xc, Yc or Xc to be used in the video datacalculating section is set separately just like the case of the slope A.

INDUSTRIAL APPLICABILITY

As discussed in the foregoing, the contrast adjusting circuit accordingto the present invention is designed so that the rise of the luminanceat low levels is prevented to prevent the image from becoming whitishwhen the contrast is raised. The present invention is also applicablefor making optimal adjustment of the contrast, coping with ever-changingconditions of the image.

1. A contrast adjusting circuit comprising a video data calculatingsection comprising a subtraction circuit for subtracting central pointdata Xc from input video data X, a multiplying circuit for multiplyingoutput data from the subtraction circuit by a slope A and an addingcircuit for adding central point data Yc to output data from themultiplying circuit to obtain output video data, designed for obtainingoutput video data Y by substituting the input video data X, the slope Aand the central point data Xc and Yc in a calculation formula,Y=A·X+Yc−A−Xc, where the input video data is given as X, X_>=_(—)0; theoutput video data after contrast adjustment as Y, Y_>=_(—)0; the slopevariably set for contrast adjustment as A, A_>_(—)0; the central pointdata as Xc and Yc, Xc_>_(—)0 and Yc_>_(—)0, an average luminancecalculating section for obtaining the average luminance level of animage during an n-frame period, n being an integer of 1 or more, and adata converting section comprising a look-up table that outputs thecorresponding central point data, Xc and Yc, by using the averageluminance, obtained by the average luminance calculating section, as anaddress, for converting the average luminance level obtained by theaverage luminance calculating section to the corresponding central pointdata, out of a plurality of predetermined central point data, whereinthe central point data Xc and Yc to be outputted from the dataconverting section are used as the central point data to be inputted tothe video data calculating section.
 2. A contrast adjusting circuitcomprising a video data calculating section designed for obtainingoutput video data Y by substituting input video data X, slope A andcentral point data Xc and Yc in a calculation formula, Y=A·X+Yc−A·Xc,where the input video data is given as X, X_>=_(—)0; the output videodata after contrast adjustment as Y, Y_>=_(—)0; the slope variably setfor contrast adjustment as A, A_>_(—)0; the central point data as Xc andYc, Xc_>_(—)0 and Yc_>_(—)0, wherein the video data calculation sectionis designed to obtain the output video data Y by a calculation formulaY=A·X+Xc·(1−A), which is introduced from the calculation formulaY=A·X+Yc−A·Xc by setting Yc=Xc.
 3. The contrast adjusting circuitaccording to claim 2, comprising an average luminance calculatingsection for obtaining the average luminance level of an image during ann-frame period, n being an integer of 1 or more, based on the inputvideo data X and a data converting section for converting the averageluminance level, obtained by the average luminance level calculatingsection, to corresponding central point data Xc and Yc out of aplurality of predetermined central point data so that the central pointdata, outputted from the data converting section, is used as the centralpoint data to be processed by the video data calculating section.
 4. Thecontrast adjusting circuit according to claim 3, wherein the video datacalculating section comprises a first multiplication circuit formultiplying the input video data X by the slope A, a subtracting circuitfor subtracting the slope A from the constant 1, a second multiplicationcircuit for multiplying the output data of the subtracting circuit bythe central point data Xc and an adding circuit for adding the outputdata of the first multiplication circuit to the output data of thesecond multiplication circuit to thereby obtain the output video data.5. The contrast adjusting circuit according to claim 3, wherein the dataconverting section comprises a look-up table for outputtingcorresponding central point data by using the average luminance obtainedby the average luminance calculating section as an address.
 6. Thecontrast adjusting circuit according to claim 4, wherein the dataconverting section comprises a look-up table for outputtingcorresponding central point data Xc by using the average luminanceobtained by the average luminance calculating section as an address.